Finding Mercury’s Water with Infrared Lasers and Neutron Counters

The satellite giveth, and the satellite taketh away. While some space-borne probes seem to confirm that earthly ice caps are shrinking, others indicate that billion-year-old ice deposits lurk in Mercury’s deep arctic shadows—which remain at a cool -173 °C despite the planet’s proximity to the sun, which can drive summer noon temperatures up to 627 °C.

Are Mercurians swiping Earth’s ice to chill their martinis? Hardly. For one thing, it’s too hard to get good vermouth up there. More important, it appears that the ice (and possibly some organic matter, which may serve as insulation, like sawdust in an old icehouse) was delivered by accommodating comets.

The MLA, built at NASA’s Goddard Space Flight Center, lights up the Mercurial surface with a 1064-nanometer (deep infrared) chromium/neodymium yttrium-aluminum garnet laser. Like orbiting laser altimeters everywhere, it flashes light off the ground below (in this case, lighting up spots about 50 meters in diameter at 400-meter intervals) and gauges distance by measuring the time until the reflected light returns. The MLA gathers additional information about the surface by alternating high- and low-power flashes. By comparing the strengths of the returning signals, it can provide more accurate estimates of the reflectance of the ground below, possibly revealing something about the surface's composition.

The result is a topographic map of Mercury based on more than 4 million individual elevation measurements—half of them including information on the nature of the ground.

The Messenger researchers were already primed to look for ice. Earth-based radar probes had previously indicated that the craters might hide deposits of frozen water—or some other captured volatile substance such as sulfur—in radar-bright areas hidden deep in the perpetual shadows of arctic craters.

The scientists found that these radar-bright areas fall into two categories. Those at the highest latitudes—where the crater-wall shadows are longest and the valleys coldest—reflect both radar and 1064-nm laser light strongly. These, the team says, are consistent with large areas of exposed ice, in layers at least several meters thick.

In slightly lower latitudes, the researchers found that the radar bright areas are often enveloped or overlaid by larger laser-altimeter-dark regions (areas that reflect little infrared). Indeed, “all craters with [radar bright] deposits and sufficient altimeter sampling show at least some [laser-altimeter dark] features in their poleward facing portions.” This suggests, say the researchers, that in these warmer craters, a thin layer of something—regolith or even comet-deposited organic compounds—may shield deeper strata of ice, protecting them from sublimation.

The altimeter findings are supported by the Johns Hopkins–built Neutron Spectrometer, which detects neutrons thrown off of atoms on Mercury’s surface as the atoms are struck by gamma rays. The emitted neutrons fall into three energy ranges: fast (energy greater than 0.5 megaelectron volt), epithermal (0.5 electron volt to 0.5 MeV), and thermal (less than 0.5 eV). Because hydrogen atoms and neutrons have such similar masses, they transfer momentum very efficiently, and the hydrogen absorbs momentum from the epithermal neutrons.

The neutron spectrometer counts the number of incoming fast, epithermal, and thermal neutrons (correcting for changes in altitude) and extrapolates the data to infer the amount of hydrogen in the surface materials. The relative proportion of epithermal neutrons among the outbound particles reveals the amount of hydrogen present. A drop of even 4 percent in the rate of incoming fast and epithermal neutrons indicates that the spacecraft is passing over hydrogen rich water.

The neutron data suggest that only half of the radar-bright regions are actually water at the surface. At the same time, the overall picture shows as much as 1000 cubic kilometers of water icebound at Mercury's poles, lying in layers “tens of centimeters thick.” Many of these layers are insulated below a superficial covering (much poorer in hydrogen compounds) 10 to 20 centimeters deep.

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